How to Size an RCBO for a Final Circuit
How do you size an RCBO for a final circuit? An RCBO combines an MCB's overcurrent tripping curve and rated current In with an RCD's earth-leakage sensitivity IΔn in one module (IEC 61009), so sizing it is really two separate calculations solved together: In must sit between the circuit's design current and the cable's current-carrying capacity, while IΔn, type, and time delay must match what's actually connected downstream. Get In wrong and the device nuisance-trips on load current or leaves the cable unprotected on overload; get IΔn or type wrong and it either misses a genuine earth fault or trips every time a variable-speed drive starts. This guide covers deriving design current, picking a standard In step, setting IΔn and type by load, checking discrimination against an upstream device, and confirming short-circuit rating.
Start With the Circuit's Design Current, Not the Catalog Page
Sizing starts upstream of the RCBO, not at it. Calculate the design current Ib from the connected load and any diversity factor, then select a cable cross-section whose current-carrying capacity Iz, after derating for ambient temperature and grouping, comfortably clears Ib. Only then does the RCBO's In get chosen, and it has to sit inside a specific window, not just "big enough."
Formula: Overload Protection Coordination — Source: IEC 60364-4-43 (overload protection of conductors)
Ib ≤ In ≤ Iz
| Symbol | Description | Unit |
|---|---|---|
| Ib | Design current of the circuit (actual load, with diversity) | A |
| In | Rated current of the RCBO's MCB element | A |
| Iz | Current-carrying capacity of the installed cable, after derating | A |
Set In too low against Ib and the MCB element trips on normal load, not on a fault. Set it too high against Iz and the cable can run hot under sustained load long before the overcurrent element reacts. The RCD side won't save you here; it only watches earth leakage, not conductor temperature.
Pick a Standard In Step, Then Check It Against the Cable
RCBOs come in fixed steps: 6, 10, 16, 20, 25, 32, 40, 50, 63 A is the common range across Schneider, ABB and Siemens residential and light-industrial lines. Round Ib up to the next available step, then confirm Iz still clears it after derating. A 32 A RCBO on a circuit wired in 2.5 mm² that only clears 27 A once you've derated for three cables bunched in a wall channel is a paper spec, not a working one. The cable fails before the breaker trips.
Curve type (B, C) sits on the MCB side of the RCBO and isn't unique to combined devices, but it still needs picking correctly: B for resistive and lighting loads, C where inrush current from motors, transformers, or some LED drivers would nuisance-trip a B-curve on switch-on.
Set IΔn for What's Actually Connected
For socket outlets and any circuit a person can touch, 30 mA is the standard personal-protection sensitivity. It trips well within the time limits IEC 61008/61009 sets for shock protection. Fixed lighting on a dedicated circuit with no exposed metalwork is sometimes run at a higher IΔn or without additional RCD protection at all, depending on the applicable wiring code, so check the local regulation before assuming 30 mA is mandatory everywhere.
What we see in the field: a 30 mA RCBO feeding six switch-mode power supplies trips on nothing in particular, repeatedly, because each supply leaks a small steady current to earth through its EMC filter and the totals add up. As a design margin, keep the sum of expected steady-state leakage from connected equipment comfortably under roughly a third of IΔn. For a 30 mA device that's under about 9 mA of background leakage before a real fault even occurs. It isn't a hard limit written into the product standard; it's a working margin manufacturers and panel builders use to avoid exactly that call-back.
Choose the Type: AC, A, F or B
Type AC detects sinusoidal AC residual current only. Most modern loads, including anything with a switch-mode power supply, a variable-speed drive, or an electronic dimmer, can superimpose pulsating or smooth DC onto the residual current, and a Type AC device can lose sensitivity or fail to see it at all. Type A adds detection of AC plus pulsating DC and is the practical default for a general-purpose final circuit today. Type F extends that to mixed frequencies from single-phase variable-frequency loads. Type B is the only type that reliably detects smooth DC residual current, required for three-phase VFDs, EV charge points, and transformerless PV inverters; see the EV charger RCD sizing guide for the charge-point case specifically.
Don't default to Type AC because it's the cheapest RCBO on the shelf. If you can't confirm what stays downstream over the life of the circuit, a socket outlet in a workshop, for instance, Type A is the safer floor, not Type AC.
Check Discrimination Against the Upstream Device
Most panels run a single upstream RCCB or main RCD ahead of several final-circuit RCBOs. Size the final circuit's IΔn and time delay so a fault on that circuit trips the RCBO, not the upstream device and everything else fed from it.
Formula: RCD Discrimination Between Final Circuit and Upstream Device — Source: manufacturer selectivity/coordination guidance under IEC 60364-5-53
IΔn(upstream) ≥ 2 × IΔn(downstream), with an added time delay upstream
| Symbol | Description | Unit |
|---|---|---|
| IΔn(upstream) | Sensitivity of the main or incoming RCD/RCCB | mA |
| IΔn(downstream) | Sensitivity of the final-circuit RCBO | mA |
A 30 mA final-circuit RCBO under a 30 mA main RCD gives no discrimination at all; both devices see the same fault, and which one opens first comes down to manufacturing tolerance, not design. Pair a 30 mA RCBO with a 100 mA or 300 mA time-delayed (S-type) device upstream, or drop the main device to an RCD sized only for fire and equipment protection and let each final circuit carry its own 30 mA RCBO. This is covered in more depth in the RCD sensitivity, type and pole selection checklist.
Confirm Poles, Inc and Backup Coordination
Poles follow the circuit, not the other way round: 2P for a single-phase final circuit, 4P for a three-phase load such as a motor feeder or a three-phase EV charge point. Check the rated conditional short-circuit current Inc, the maximum prospective fault current the RCBO can withstand when backed by a specified upstream device, commonly quoted as 6 or 10 kA with a stated backup fuse or MCCB. If the prospective fault current at that point in the installation exceeds the RCBO's own breaking capacity, the upstream device, not the RCBO, has to be rated to clear it, and the coordination between the two needs to be read from the manufacturer's tables, not assumed.
Sizing by Circuit Type: A Quick Reference
The table below is a starting point, not a substitute for the calculation above. Actual In always comes from Ib and Iz on the specific installation, and actual IΔn always comes from what's connected.
| Circuit Type | Typical In | Typical IΔn | Type |
|---|---|---|---|
| General socket outlets | 16–32 A | 30 mA | A |
| Fixed lighting | 6–10 A | 30 mA where required, or none | A |
| Single-phase EV charge point | 32 A | 30 mA + 6 mA DC (RDC-DD), or Type B alone | A + RDC-DD, or B |
| Three-phase motor/VFD feeder | Per motor full-load current | 30–300 mA depending on duty | B |
Cross-check against the wider RCD sensitivity ratings breakdown and the RCD types AC, A, F and B explanation before finalizing a spec sheet. For terminology on how an RCBO differs from a plain RCCB plus a separate RCD block, see the differences between MCB, RCD, RCCB and RCBO. Some panel builders still fit an MCB plus a clip-on RCD block instead of a one-piece RCBO, mainly so the earth-leakage function can be reset or replaced independently of the overcurrent function; it costs an extra module width for that flexibility.
Frequently Asked Questions
What In rating should I use for a standard 32 A socket circuit?
Start from the actual design current Ib, not the circuit's nominal description. Confirm the installed cable's derated capacity Iz clears the chosen In. A 32 A RCBO only makes sense if the cable, after derating for grouping and ambient temperature, still clears 32 A.
Should a final circuit RCBO be 30 mA or 100 mA?
30 mA for any circuit a person can touch: sockets, portable equipment, most general circuits. 100 mA and above are fire and equipment-protection sensitivities, used further upstream where personal shock protection isn't the primary concern, or on circuits with high inherent background leakage.
Do I need Type A or Type B for a final circuit?
Type A covers AC plus pulsating DC and suits most general sockets and electronic loads. Type B is required specifically where smooth DC residual current can occur: three-phase VFDs, EV charge points without a separate RDC-DD device, and transformerless PV inverters.
How do I check discrimination between a final-circuit RCBO and an upstream RCD?
Compare IΔn ratings. The upstream device should be rated at roughly twice the downstream RCBO's IΔn or more, ideally combined with a time delay from an S-type device upstream. Manufacturer coordination tables confirm the exact pairing for a given product range.
Can one RCBO replace a separate MCB and RCD block on a final circuit?
Yes, that's the point of the RCBO format. It combines both functions in one module, at the cost of losing the ability to reset or replace the RCD function independently of the overcurrent function; some panel builders still prefer the separate MCB-plus-block approach for that reason.
Conclusion
Sizing an RCBO for a final circuit is two calculations, not one: In from Ib and Iz on the overcurrent side, IΔn and type from the connected load's leakage behavior on the residual-current side, checked against discrimination with whatever sits upstream. Skip either half and the device either fails to protect the circuit it's rated for or trips on things that were never a fault in the first place. For the broader selection framework across sensitivity, type and poles, see the RCD selection checklist, and for the full engineering background start with the RCD protection guide. Browse the current range of RCBOs and general residual current devices for available In, IΔn and type combinations in stock.
